One example of the frequency analyzing apparatus of this kind and one example of the spectrum analyzer including this frequency analyzing apparatus built therein are shown in FIG. 8. Further, in the illustrated examples, a part that detects a signal to be measured and analyzes the frequency of the detected signal will be referred to as “frequency analyzing apparatus” and the whole construction including the frequency analyzing apparatus and a display that displays the result of analysis carried out by the frequency analyzing apparatus will be referred to as “spectrum analyzer”.
As shown in FIG. 8, the prior art frequency analyzing apparatus 100 comprises a signal extracting device 11 that extracts a signal existing in a specified frequency band X to be measured (see FIG. 9A), an analog-to-digital converter (hereinafter, referred to as A/D converter) 12 connected to the signal extracting device 11, a frequency analyzing part 13 that analyzes the frequency of digital data outputted from the A/D converter 12, a buffer memory 16 that stores the result of frequency analysis outputted from the frequency analyzing part 13, a clock source 14 that applies a clock signal to the A/D converter 12, and a frequency divider or demultiplier 15 that frequency divides a clock signal supplied thereto from the clock source 14 and supplies the frequency divided clock signal to both the frequency analyzing part 13 and the buffer memory 16.
On the other hand, the spectrum analyzer 200 comprises the frequency analyzing apparatus 100 constructed as described above, and a display 17 that displays the result of analysis carried out by the frequency analyzing apparatus 100.
The signal extracting device 11 may be constructed by a high or radio frequency receiver in case a signal to be measured is, for example, a radio wave. In addition, in case a signal to be measured is, for example, a signal obtained from a signal transmission line (a relatively low frequency signal), the signal extracting device 11 may be constructed by a bandpass filter. It is the construction of the frequency analyzing apparatus 100 in case a signal to be measured is a radio wave that is shown in FIG. 8. In such case, an antenna AN is used as a sensor that acquires a signal to be measured, and the antenna AN is connected to the signal extracting device (high frequency receiver in this example) 11. In other words, the antenna AN is connected to an input terminal of the high frequency circuit of a spectrum analyzer that will be usually used, for example, in case of analyzing the frequency of a high frequency signal. Further, in case that a specified frequency band is relatively narrow, and that the signal extracting device 11 is utilized as a means that merely extracts from an input signal thereinto a signal existing in the specified frequency band, the spectrum analyzer makes use of the signal extracting device 11 as a bandpass filter without frequency sweeping.
The high frequency circuit of a spectrum analyzer that will be usually used, in general, down converts in frequency high frequency signals S1, S2 existing in a frequency band to be measured (high frequency band) X shown in FIG. 9A into intermediate frequency signals IF1, IF2 lower in frequency than those of the high frequency signals S1, S2, respectively, as shown in FIG. 9B by use of a super heterodyne receiving system, and outputs them.
Therefore, the signals to be measured S1, S2 existing in the frequency band to be measured X are extracted by the signal extracting device 11 utilized as a bandpass filter in this case, and are inputted into the A/D converter 12 after they are down converted in frequency into the intermediate frequency signals IF1, IF2. In order to abbreviate the explanation, the following description will be given on the assumption that the signals to be measured S1, S2 are inputted into the A/D converter 12 from the signal extracting device 11 without frequency conversion.
The A/D converter 12 converts the signals to be measured S1, S2 into digital signals in synchronism with a clock signal supplied from the clock source 14 to form a series of digital data. The signals to be measured S1, S2 that have been converted into digital data series respectively by the A/D converter 12 are inputted into the frequency analyzing part 13 in which amplitude data of the signals to be measured S1, S2 are computed for frequency of each of sample points in the frequency band to be measured X and frequency analysis is performed.
As the frequency analyzing part 13, a device or apparatus utilizing the fast Fourier transform (FFT) that is already well known (hereinafter, referred to as fast Fourier transform device) may be used, for instance. In case of using the fast Fourier transform device, it is possible to compute, from a signal with which the signals to be measured S1, S2 intermingle, amplitude data of the signals to be measured S1, S2 for frequency of each sample point in the frequency band to be measured X and to perform frequency analysis thereof.
In the frequency analyzing part 13, computation or operation for analyzing the frequency for each of the signals S1 and S2 to be measured is carried out each time the A/D converter 12 outputs, for example, S digital sample values to the frequency analyzing part 13 and at the time point (timing) that the S-th last digital sample value is outputted from the A/D converter 12. Further, in case the A/D converter 12 outputs S digital sample values, the frequency divider 15 forms a 1/S frequency divider or demultiplier that divides the frequency of the clock signal by S.
One frequency analysis computation or operation results in that M data latches 16A, 16B, 16C, . . . , 16M prepared in the buffer memory 16 have respective amplitude data stored therein corresponding to frequencies at the every sample point in the frequency band to be measured X. In this example, since there are S sample points, the number M of the data latches may be an integer equal to or greater than S (M≧S). When the next frequency analysis computation or operation with respect to the next S digital sample values is started in the frequency analyzing part 13, the amplitude data already latched in the buffer memory 16 are sequentially sent out to the display 17 and stored in an image or picture memory of the display 17.
One example of the result of the frequency analysis stored in the image memory of the display 17 is shown in FIG. 10 in diagrammatical form. In this figure, a lateral axis F shows frequency, a vertical axis Y shows amplitude, and a back-and-forth direction (depth direction) axis T shows time. It will be easily understood from the waveforms in FIG. 10 that the result of the frequency analysis for the signals to be measured S1 and S2 at the timing t1, the result of the frequency analysis for the signals to be measured S1 and S2 at the timing t2, have been stored in the image memory, respectively.
The display 17 reads out these results of the frequency analyses at respective timings t1, t2, t3, . . . one at a time and indicates it on its display screen to measure the amplitudes of the signals S1 and S2 to be measured, the center frequencies, bandwidths, etc. of the signals S1 and S2 to be measured. Thus, the frequency analyses for the signals to be measured S1 and S2 existing in the frequency band to be measured X can be carried out.
The results of the frequency analyses shown in FIG. 10 illustrate ideal states. In reality, supposing that the signals to be measured S1 and S2 are radio waves generated from portable telephones or mobile (cellular) phones for example, amplitude values thereof are in a state of disorder such that they cannot be distinguished from noises because the radio waves generated from the portable telephones are spread in frequency. Particularly, in case the level of a radio wave is low, it is utterly impossible to discriminate between the radio wave and a noise. For this reason, there is a serious defect in the prior frequency analyzing method and apparatus that the presence of a radio wave the frequency of which is spread as in a portable telephone cannot be reliably detected.